Soil settling is a primary cause of cracks in a concrete slab. Soil compaction increases the density of the soil, making it less prone to moisture and settling. To compact soil correctly for a concrete slab may involve numerous passes with a tamping machine and testing the soil's density before pouring the concrete.
Things You'll Need
- Steel tine rake
- Fill dirt
- Water hose
- Hand tamp
- Soil density testing machine
- String line
- Tape measure
Use a steel-tined rake to rake the soil level. Remove any rocks that may be in the soil. Add more clean fill dirt to any areas that are low, or remove high spots with your shovel.
Spray down the slab area with water. Don't spray so much water that puddles form, but you do want the soil to have an even, dark brown coloring. Aim your hose spray into the air, about 5 inches over the surface of the soil as you spray, rather then spraying the water directly into the soil.
Compact the soil with a mechanical compactor for large areas or a hand tamp for small ones. Work in a "row" pattern, as if you were mowing a lawn, compacting the soil from one side of the forms to the other and starting a new row next to the old. If you are using a mechanical compactor, follow the manufacturer's instructions for operating the machine. If you are using a hand tamp, walk in short half-steps. With each step, raise the tamp so the cast iron end is waist high and slam it down into the ground. Make sure that each strike of the tamp overlaps the print in the soil from the last. You want the entire surface of the soil to be smooth and dense; you should be able to walk over it without leaving noticeable prints.
Test the soil density with a testing machine, as per the instructions included with it. Drive the test spike into the ground to the proper depth with a hammer to get the best results. If you do not have access to a density machine, you can test it by hand. If you the soil passes the density test, you are ready to install your rebar or mesh and pour your concrete. If the soil fails, repeat the compacting process and retest the density.
Tips & Warnings
- Do two things at once to speed up the preparation of your slab. As you compact the soil, run a string line from form to form at the height of the slab. Then measure down from the string line to the level of the ground to make sure that you have the soil at the right level to allow the concrete to achieve the right thickness for the slab. This way you can add or take away soil as needed and compact it right then and there rather then having to go back and do it.
- Only licensed professionals should use soil density testing machines. The machines utilize a radioactive element to conduct the test, and this can create a hazardous situation if not handled correctly.
How to Build on Fill Dirt
Fill dirt is commonly added to building sites to create a level area on which to build. While it is an economical...
How to Compact Fill Dirt for Building
Compacting fill dirt for proper building will save you costly headaches down the road. An improperly compacted fill will cause the building...
How to Compact Soil With a Water Hose
Gardeners and backyard home improvement enthusiasts alike can find themselves needing a quick and cost-effective way of tamping down soil before or...
How to Backfill a Trench & Settle Dirt
If you have dug a trench to repair a pipe, install outdoor lighting or for some other project, you must eventually refill...
How to Pack Dirt for Concrete
Pouring concrete for slabs, driveways, walkways, patios or any other project necessitates tightly packed soil to ensure the integrity of the concrete....
How to Tamp a Fence Post
The best way to secure a fence post into the ground is by filling the hole around the post with cement. However,...
How to Loosen Compacted Soil
Compacted soil is pressed together to form a solid mass with little air space between the particles. Plants often have a hard...
Dos & Don'ts of Laying Sod
Installing sod instead of planting grass seed allows you to have a green lush lawn right away. Keeping the sod green and...
Soil Compaction Testing Methods
Whether you want to cultivate a lush backyard garden or construct an addition to your home, soil compaction is an important factor...